Tunable circuits



June 1, 1943. A C, STOCKER 2,320,483

TUNABLE CIRCUIT Filed Sept, 30, 1941 llll a F. j 1.5/ 49 CURE 72F VEL nveutor l Cttorncj Patented June 1, 1943 i ri OFFICE TUNABLE CIRCUTS Arthur C. Stocker, Collingswood, N. J., assigner to Radio Corporation of America, a corporation of Delaware 9- Claims.

This invention relates to tunable circuits, and more particularly to circuits employing trimmers.

`oscillators andother similar tunable circuits are frequently equipped with direct reading dials and means for trimming the operating frequency to agree with these dials. Thus, variable frequency oscillators are commonly equipped with a widely variable reactance which acts as a tuning control, and a small variable reactance for adjustment ofv the tuningl control, the'latter reactance being known as a trimmer. The-amount of control given by the trimmer depends upon the operating frequency and upon' the percentage change in reactance, sov that-the trimmers are more effective at one end of the tuning range than at the other.

The Worst example of this may be foundy when the main `and trimmer reactors are of the same sign (inductive or capacitive). Assuming a frequency change of 3:1 .by the main, yor tuning, reactor, then, if the trimmer is l percent of the mainl reactor at its maximum end, it will be 9 percent at the other end, so that the percentage frequency change available varies by \/9, or 3, over the operating band. Further, the highest percentage change occurs at the highest frequency, so that the actual cyclesfchange available'is 3X3; or 9, at the high frequency end for every one available at the low frequency end.

With the main and trimmer reactors of opposite sign, the percentage change duev to the trimmer remains the same, but the actual variation in cycles is proportional to frequency, so that, if a 3:1 band is covered, the ltrimmer is three times `as' eiiective at the high frequency end as it is at the low frequency endf.

The primary object lof my present invention is to provide a novel't'uning circuit incorporating an adjusting trimmer which is not subject to the foregoing defect'.

More particularly, it is an object of my present invention to provide a novel trimmer arrangement wherein the action of the primary adjusting trimmer will be constant over the operating range of the apparatus.

It is also an object of my present invention to lprovide :a novel trimmer network which is simple in construction, inexpensive in cost and highly eicient in use.

In accordance with my invention, I provide a trimmer tuning network in Circuit with one of the elements of the tunable circuit, the network having two parallel' branch circuits. Each of lthe branch circuits includes two serially connected inductors, one of the inductors of one of the branch circuits being variable and constituting the trimmer inductor, and one of the inductors in the other branch circuit having a fixed value equal approximately to the average value of the trimmer inductor. The other two inductors, one in each of the branch circuits, are arranged to cooperate as a pair and have a common core which is connected to the main tuning element. Thus, the two inductors acting as a ypair constitute a balance coil the function of which is to 4control' the yeffectiveness, of the trimmer without changing the total inductance of the network. By making thev value of the fixed inductor in the network equal to the average value of the trimmer inductor, the balance coil is enabled to perform its function.

The novel features that I consider cha-racteristic of my invention are .set forthwith particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together withl additional objects and advantages thereof, will best be understood from the following description, when read in connection with the accompanying drawiner wherein Figure 1 isY av Wiringv diagramv ofy an oscillator embodying` a trimmer tuning network in; accordance with this invention, n

Figure' 2-is an elevational view of the bala-nce coil structure, and

Figure 3 is a 'set of curves illustrating the effectiveness of the individual inductors constituting the balance coil in' the various positions of the common core therefor` Referring more particularly to the drawing, wherein similar reference characters designate corresponding parts throughout, there is shown, in Fig. l, an oscillator tube l having. a feedback winding 3 in the platey circuitv 5 thereof, the winding 3 being coupled to. the grid main inductor l. The main induct-or l cooperates with a variable capacitor 9 in the grid circuit I l of the oscillator l toprovide a tunable circuit, the capacitor 9 yserving as a main tuning element for tuning the oscillator.

In series with the main inductor l is a trimmer network I3 having. two parallel branches i3d and i319. The branch i3d has a pair of serially connected variable inductors l5 and I1, and the branch |317l has a pair of serially connected inductors r9 and 2l of which the inductor I'S is variable and the inductor 2| is fixed. The inductor il constitutes the trimmer reactance for yadjusting the range of. the main tuning `capacitor 9, and it may be manually adjusted by means of a knob 23 or the like. The inductor 2l has a value equal approximately to the average value of the trimmer inductor Il.

The inductors I5 and I9 are preferably wound on a common form or tube of insulating material and have a common iron core 21 so arranged that when the core 21 is fully within the winding I5, it is out of the winding I9, land vice versa. Thus, when the core 21 is fully within the winding I5 and entirely out of the winding I9, the inductor I5 has maximum inductance, as shown by the curve I of Fig. 3, and the inductor I9 has minimum inductance, as shown by the curve II of .this gure. Conversely, with the core 21 fully inserted in the winding I9 and out of the winding I5, the latter has minimum inductance and the former has maximum inductance, yas shown by the same curves. and I9 are arranged in parallel-aiding relation, and their combined eiiect is shown by the curve III of Fig. 3. The common core 21 is connected to the control element of the timing capacitor 9 for unitary action therewith in response to a control knob 29 having a pointer 3| arranged to operate over a scale 33, and for this purpose, the core 21 may have connected thereto one end of an operating rod the other end of which carries a pin 31. received in a cam slot 39 of a cam 4I fixed to the tuning shaft 43 on which the knob 29 is mounted.

As noted heretofore, the capacitor 9 constitutes the main tuning reactance and, together with the inductor 1, may correspond to the tank circuit customarily employed in connection with an oscillator. The trimmer inductor I1 may be adjusted by means of the knob 23 independently of the knob 29 to provide the desired compensation. The effectiveness of the primary inductor I1 is controlled by the cooperating inductors I5 and I9 which constitute a balance coil. As the core 21 is operated simultaneously with adjustment of the tuning condenser 9, it will either decrease or increase the reactance of the inductor I5 and correspondingly increase or decrease the reactance of the inductor I9, as the case may be, thereby changing the reactance of the individual branches I3a, and I3b. However, the total reactance of the network is not changed, and the effectiveness of the trimmer inductor therefore remains constant throughout the tuning range of the oscillator circuit.

Although I have shown land described but a single embodiment of my invention, it will be apparent to those skilled in the art that other modications are possible. I desire, therefore, that my invention shall not be limited except insofar as is made necessary by the prior art and by the spirit of the appended claims.

I claim as my invention:

1. A tunable circuit comprising a fixed reactance element and a variable reactance element. and a trimmer tuning network in circuit with one of said elements, said network comprising two parallel branch circuits each having two series connected inductors, and means for oppositely varying the inductance of a cooperating pair of The windings or inductors I5 said inductors of which one is in one of said naad/ies ance element and a variable reactance element, and a trimmer tuning network in circuit with one of said elements, said network comprising two parallel branch circuits each having two series connected inductors, means for oppositely varying the inductance of a cooperating pair of said inductors of which one is in one of said branch circuits and the other is in the second of said branch circuits for varying the reactance of each of said branch circuits while maintaining the overall reactance of said network substantially constant, and means for varying the inductance of the second of said two inductors in one of said branch circuits.

3. A tunable circuit comprising a fixed reactance element and a variable reactance element, and a trimmer tuning network in circuit with one of said elements, said network comprising two parallel branch circuits each having two series connected inductors, one of said inductors in one of said branch circuits and one of said inductors in the second of said branch circuits cooperating as a pair, means for oppositely varying the inductance of each of said pair of cooperating inductors in predetermined relation to said variable reactance element, and means far varying the inductance of the other of said inductors in one of said branch circuits.

4. A tunable circuit comprising a fixed reactance element and a variable reactance element, and a trimmer tuning network in circuit with one of said element, said network comprising two parallel branch circuits each having two series connected inductors, one of said inductors in one of said branch circuits and one of said inductors in the second of said branch circuits cooperating as a pair, means for oppositely varying the inductance of each of said pair of cooperating inductors in response to tuning variations of said variable reactance element whereby to vary the reactance in each of said branch circuits without substantially changing the overall reactance of said network, the second of said inductors in one of said branch circuits constituting a. trimmer inductor, and means for varying the inductance of said trimmer inductor independently of the tuning variations of said variable reactance element for modifying the frequency response of terized in that the second inductor in the second of said branch circuits is constituted by a tixedinductor having a value equal substantially to the average value of said trimmer inductor.

6. A tunable circuit comprising a xed inductive reactance element and a variable capacitive reactance element, and a trimmer tuning network in series with said fixed element, said network comprising two parallel branch circuits one having a iirst variable inductor and a .second variable inductor serially connected to each other, and the other of said branch circuits having a third inductor which is Variable and a fourth inductor which is fixed, said third and fourth inductors being also serially connected to each other, said first and third variable inductors cooperating as a pair, means for oppositely varying the inductance of said first and third inductorsy in response to tuning variations of said variable capacitive reactance element whereby to vary the reactance in each of said branch circuits without substantially changing the overall reactance of said network, said second variable inductor constituting a trimmer inductor, and means forV varying the lnductance of said trimmer inductor independently of the tuning variations of said variable capacitive reactance element for modifying the frequency response of the tunable circuit as determined by said variable capacitive reactance element, said fourth inductor having a value equal substantially to the average value of said trimmer inductor.

7. In a tunable signal circuit, the combination with inductive and capacitive tuning elements, one of which constitutes a variable main tuning element, of a movable core trimmer inductor connected in said circuit and having a frequency variation range equal approximately to a predetermined percentage of the frequency variation range of said circuit at one end of the tuning range, and a balancing network in circuit with said trimmer inductor, said network including a pair of oppositely variable tuning elements connected to and variable simultaneously with said main tuning element for maintaining the effective range of said trimmer inductor substantially constant throughout the tuning range of said circuit.

8. The invention set forth in claim 7 characterized in that said network includes a second movable core inductor in series with said trimmer inductor and forming therewith one branch of said network, and characterized further in that said network also includes a second branch in parallel relation with said rst named branch, said second branch comprising a third movable core variable inductor and a xed inductor in series therewith, and said second and third movable core inductors constituting said pair of variable tuning elements.

9. The invention set forth in claim 7 characterized in that said network includes a second movable core inductor in series with said trimmer inductor and forming therewith one branch of said network, and characterized further in that said network also includes a second branch in parallel relation with said rst named branch, said second branch comprising a third movable core variable inductor and a xed inductor in series therewith, and said second and third movable core inductors constituting said pair of variable tuning elements.

ARTHUR C. STOCKER. 

